The present invention relates to a cooking vessel having improved structural and heat-transfer qualities, as well as to a method for producing such cooking vessels. In particular, such cooking vessels are one-piece, preferably being formed from stainless steel, cup-shaped blanks.
U.S. Pat. No. 3,556,032 describes a method permitting manufacture of a very thin-walled container with a curved bottom by a rolling process. For this purpose, an arrangement of four balls is used, the balls facing one another in pairs and being spring-mounted in the radial direction. The balls are located in spherical segments so that they are not peripherally rotated when the rolling process is started by rotating a central male die accompanied by the simultaneous axial movement of the rolling or curling tool. The bottom is also worked by a rolling tool provided around the edges between the bottom and the casing. The wall thickness is somewhat greater in this area than in the remainder of the casing. With this method and apparatus it is not possible to manufacture cooking vessels with a thick, and in particular, completely flat bottom from very hard materials, such as stainless steel.
Methods are also known for the manufacture of vessels, particularly cooking vessels, from softer materials such as aluminium, in which the casing is shaped by a rolling process using pressure rollers and its wall thickness is reduced (German Pat. No. 508,658, U.S. Pat. No. 2,160,975).
German Pat. No. 879,797, Offenlegungsschrift 2,452,374 and Swiss Pat. No. 297,494 disclose the deep drawing of such vessels. According to Offenlegungsschrift 1,652,630, the upper edge of the casing can be flanged by a rolling process.
None the above-described methods gives satisfactory results. In most methods, it is necessary to re-turn the bottom of the cooking vessel, because the latter is not sufficiently flat. Surface machining is also generally subsequently necessary, particularly in the case of deep drawn products, in order to remove machining marks. The flatness of the bottom (not to be confused with smoothness of surface texture) is of the greatest importance for a good heat transfer between the electrical hotplate and the cooking vessel and consequently for the efficiency of the cooking process.